This invention relates to an ion exchange purification method of an aqueous caprolactam solution. Particularly, the present invention relates to an ion exchange purification method of an aqueous caprolactam solution, which comprises purifying a crude aqueous caprolactam solution obtained in the intermediate stage in a process for producing caprolactam and laurolactam, by subjecting a mixture of cyclohexanone oxime and cyclododecanone oxime to Beckmann rearrangement in the presence of sulfuric acid and fuming sulfuric acid, by use of a suitable combination of ion exchange resins.
As the process for producing caprolactam and laurolactam, a process which comprises oximizing a mixture of cyclohexanone and cyclododecanone, subjecting a mixture of the cyclohexanone oxime or a salt thereof and the cyclododecanone oxime or a salt thereof produced to Beckmann rearrangement in the presence of sulfuric acid or fuming sulfuric acid, subsequently neutralizing the product with ammonia gas or ammonia water to obtain a lactam mixture, extracting the lactam mixture with an organic solvent immiscible with water or extracting the lactam oil layer separated from the aqueous ammonium sulfate by layer separation of the lactam mixture (hereinafter called lactam oil) with an organic solvent immiscible with water to obtain a first extract containing the lactam component, further reverse extracting the organic solvent extract (first extract) with water to transfer caprolactam to the aqueous layer (second extract), while having laurolactam remain in the organic solvent layer (hereinafter called the co-lactamization method) has been known in the art (for example, Japanese Patent Publication No. 7254/1971).
On the other hand, it has been well known in the art that some of the caprolactams to be used as the starting material for polyamides are required to be of extremely high purity, and various purification methods such as recrystallization, treatment with ion exchange resins, oxidation purification, etc. have been proposed. Among them, treatment with ion exchange resins has been known to be an effective method (for example, Japanese Patent Publication No. 6217/1968).
However, in the simultaneous production process of caprolactam and laurolactam by the co-lactamization method, in the aqueous caprolactam solution obtained by water extraction (second extract) of the organic solvent extract (first extract) containing the above lactam components, there exist anionic surface active substances specific in the co-lactamization such as alkyl sulfates due to higher alkyl alcohols such as linear or branched dodecyl alcohol, etc. contained in cyclododecanone which is the starting material. Also, the impurities which are water-soluble colored impurities formed in the Beckmann rearrangement and migrated into the organic solvent layer by droplet accompaniment, etc. in the organic solvent extraction (first extraction) of the lactam oil will be migrated into the aqueous layer in water extraction (second extraction). Accordingly, the present inventors have previously proposed to remove the impurities by treating an aqueous caprolactam solution (said second extract) containing these impurities with an ion exchange resin (for example, Japanese Patent Application No. 151572/1990), but alkyl sulfates such as dodecyl sulfate, etc. are strongly adsorbed on strongly basic anion exchange resins to such extent as almost impossible of desorption in ion exchange resin purification, whereby there is involved the problem that regeneration use of ion exchange resins can be done with difficulty.